Photolithography or optical lithography is a process used, inter alia, in semiconductor device fabrication to transfer a pattern from a photomask (sometimes called reticle) to the surface of a substrate. Such substrates are well known in the art. For example, silicon, silicon dioxide and aluminum-aluminum oxide microelectronic wafers have been employed as substrates. Gallium arsenide, ceramic, quartz and copper substrates are also known. The substrate often includes a metal coating.
Photolithography generally involves a combination of substrate preparation, photoresist application and soft-baking, radiation exposure, development, etching and various other chemical treatments (such as application of thinning agents, edge-bead removal etc.) in repeated steps on an initially flat substrate. In some more recently-developed techniques, a hard-bake step is implemented after exposure and prior to development.
A cycle of a typical silicon lithography procedure begins by applying a layer of photoresist—a material that undergoes a chemical transformation when exposed to radiation (generally but not necessarily visible light, ultraviolet light, electron beam, or ion beam)—to the top of the substrate and drying the photoresist material in place, a step often referred to as “soft baking” the photoresist, since typically this step is intended to eliminate residual solvents. A transparent plate, called a photomask or shadowmask, which has printed on it areas that are opaque to the radiation to be used as well as areas that are transparent to the radiation, is placed between a radiation source and the layer of photoresist. Those portions of the photoresist layer not covered by the opaque areas of the photomask are then exposed to radiation from the radiation source. Exposure is followed by development. In some cases, exposure is followed by a post-exposure bake (PEB), which precedes the development. Development is a process in which the entire photoresist layer is chemically treated. During development, the exposed and unexposed areas of photoresist undergo different chemical changes, so that one set of areas is removed and the other remains on the substrate. After development, those areas of the top layer of the substrate which are uncovered as a result of the development step are etched away. Finally, the remaining photoresist is removed by an etch or strip process, leaving exposed substrate. When a “positive” photoresist is used, the opaque areas of the photomask correspond to the areas where photoresist will remain upon developing (and hence where the topmost layer of the substrate, such as a layer of conducting metal, will remain at the end of the cycle). “Negative” photoresists result in the opposite—any area that is exposed to radiation will remain after developing, and the masked areas that are not exposed to radiation will be removed upon developing.
The need to make circuits physically smaller has steadily progressed over time, necessitating inter alia the use of light of increasingly shorter wavelengths to enable the formation of these smaller circuits. This in turn has necessitated changes in the materials used as photoresists, since in order to be useful as a photoresist, the material should not absorb light at the wavelength used. For example, phenolic materials which are commonly used for photolithography using light of wavelength 248 nm wavelength are generally not suitable for use as photoresists for light of 193 nm, since these phenolic materials tend to absorb 193 nm light.
At present, it is desired to use light in the extreme UV range (13.5 nm or shorter) for photolithography of circuits having line widths of 32-20 nm. Many of the materials which would be suitable for use as positive photoresists in this range are polymers which contain acidic groups in protected form, such as tert-butoxycarbonyl (t-BOC) protected forms of polymers derived from polyhydroxystyrene or t-butylacrylate polymers. Following the “soft bake” of the photoresist, exposure of the masked photoresist to radiation and, if necessary, post-exposure bake should result in deprotection of polymers in the areas which were not covered by the opaque portions of the mask, thus rendering these areas susceptible to attack by base, to enable the removal of these areas in the development step. In order to achieve this result, it has been proposed to utilize “chemically amplified” photoresists. The idea is to include in the photoresist an amount of a thermally stable, photolytically activated acid precursor (sometimes called a “photoacid generator” or “PAG”), so that upon irradition acid will be generated which can deprotect the irradiated portions of the positive photoresist polymer, rendering them susceptible to base attack.
In a variation on the chemical amplification technique, it has been proposed to include in the resist composition a photoacid generator, as well as an acid precursor (sometimes referred to as an “acid amplifier”) which is (a) photolytically stable and (b) thermally stable in the absence of acid but thermally active in the presence of acid. In such systems, during radiation exposure the PAG generates acid, which then during post-exposure bake acts as a catalyst to activate the acid-amplifier. Such systems are sometimes referred to in the literature as “acid amplifier” systems, since the catalytic action of the photolytically-generated acid on the second acid precursor during post-exposure bake results in an effective number of acid molecules which is higher than the number of photons absorbed during radiation exposure, thus effectively “amplifying” the effect of exposure and amplifying the amount of acid present.
Similarly, the use of PAGs and acid amplifiers in negative resists has been proposed. In these cases, the acid generated makes the areas of resist exposed to radiation less soluble in the developing solvent, usually by either effecting or catalyzing cross-linking of the resist in the exposed areas or by changing the polarity or hydrophilicity/hydrophobicity in the radiation-exposed areas of the resist.
Among the difficulties encountered in trying to implement chemical amplification photoresists systems is “outgassing”, a process whereby, as a result of acid formation, gas is generated, leading to volatile compounds that can leave the resist film while the wafer is still in the exposure tool. Outgassing can occur under ambient conditions or under vacuum as is used with extreme ultraviolet (EUV) lithography. Outgassing is a problem because the small molecules can deposit on the optics (lenses or mirrors) of the exposure tool and cause a diminution of performance. Furthermore, there is a trade-off between resolution, line-width roughness and sensitivity. A resist's resolution is typically characterized as the smallest feature the resist can print. Line width roughness is the statistical variation in the width of a line. Sensitivity is the dose of radiation required to print a specific feature on the resist, and is usually expressed in units of mJ/cm2. Moreover, hitherto it has proven difficult to find acid precursors which display the requisite photostability, thermal stability in the absence of acid, and thermal acid-generating ability in the presence of acid, and which generate acids which are sufficiently strong so as to deprotect the protected resins used in photolithography.
Thus, although some acid amplifier systems have been proposed for use in photolithography using 248 nm light, there remains a need for acid amplifier systems which may be used in photolithography, particularly for use in extreme UV (13.5 nm) or electron-beam lithography.